[Bug c++/57527] New: [C++11] Nested variadic templates cause internal compiler error

[markus.mayr at outlook dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57527

            Bug ID: 57527
           Summary: [C++11] Nested variadic templates cause internal
                    compiler error
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: markus.mayr at outlook dot com

I do not know whether the following program is supposed to compile according to
the standard. I only know that it compiles using clang and that it causes an
internal compiler error. I use a variadic template constructor within a
variadic template class and expand both parameter packs simultaneously.

struct Z {};
template <class Arg, unsigned int N>
struct X {};
template <class... Args>
struct Y 
{
    template <unsigned int... N>
    Y( X<Args, N>... xs )
    {} 
};  

int main( int argc, char **argv )
{
    X<Z,1> x;
    Y<Z> y( x );
    return 0;
}   

Instantiating the constructor causes an internal compiler error:

t.cpp: In function 'int main(int, char**)':
t.cpp:16:25: internal compiler error: in unify, at cp/pt.c:16369
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://trac.macports.org/newticket> for instructions.

If x is replaced by a temporary, the code compiles, i.e. replacing main() by

int main( int argc, char **argv )
{
    Y<Z> y( X<Z,1> x );
    return 0;
}

compiles. Turning x into an rvalue reference, however, does not compile.

I added the code as an attachment. My compiler version is g++-4.7.2, but I am
currently compiling a more recent version of g++ (4.8.0) to test this. I tested
the code on Mac OS X and Debian GNU/Linux. As already mentioned, clang++ seems
to accept the code. As a side note, I would also be interested in a workaround
for enumerating arguments in a parameter pack provided that there might be
arguments of the same type in the pack.

I hope that this bug is new to you.